Solid-state image pickup apparatus and image pickup system

ABSTRACT

A solid-state image pickup apparatus includes a reading unit having a plurality of pixels connected thereto, holding signals from the pixels, and a control unit capable of controlling operations of the pixels and reading unit. The control unit controls the pixels and reading unit in a first operation mode without addition, in a second operation mode in which signals from aa of the pixels are added, aa being an integer greater than one, and in a third operation mode in which signals from bb of the pixels are added, bb being an integer greater than aa. The reading unit includes a holding unit having a capacitance value of C, and the holding unit includes a first capacitor having a capacitance value of C/bb and a second capacitor having a capacitance value of C/p, p being a common multiple of aa and bb.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/558,330,filed on Sep. 11, 2009, which claims priority from Japanese PatentApplication No. 2008-241013 filed Sep. 19, 2008, which are herebyincorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image pickup apparatus,and in particular to a solid-state image pickup apparatus that iscapable of adding signals from pixels.

2. Description of the Related Art

In recent years, solid-state image pickup apparatuses have been used inincreasingly wide application areas and are also used in, for example,digital still cameras (hereinafter called DSCs). There has been activecompetition in DSCs from the view point of increasing resolution byincreasing the number of pixels. There exist products with a resolutionhigher than ten million pixels. In addition to higher resolution, amovie function is also required. For instance, there exist productscapable of outputting VGA size data (640×480 pixels) at a rate of 30frames per second (30 fps). Demand for higher output rates is expected.

In an operation mode which requires a high output rate, an operationcalled pixel addition is known. Pixel addition allows the number ofsignals output from a solid-state image pickup apparatus to be decreasedby adding signals from a plurality of pixels, while suppressingdegradation of image quality.

Japanese Patent Laid-Open No. 2004-304771 discloses a technique thatrealizes pixel addition in a direction along a column. FIG. 16 is FIG. 2of Japanese Patent Laid-Open No. 2004-304771, showing a signalprocessing unit corresponding to pixels of two columns. Here, referencenumerals have been changed for ease of description. In FIG. 16,reference numerals 1200 a to 1200 c denote sampling capacitors,reference numeral 1570 denotes a horizontal signal line, referencenumeral 1210 denotes a horizontal signal line capacitor, referencenumeral 1600 denotes sample hold transistors, reference numeral 1610denotes clamp capacitors, and reference numerals 1630 a to 1630 c denotesampling transistors. Reference numeral 1640 denotes clamp transistors,and reference numeral 1650 denotes column selection transistors. In FIG.16, one column is provided with the three sampling capacitors 1200 a to1200 c connected in parallel, which are independently selectable by thesampling transistors 1630 a to 1630 c.

In Japanese Patent Laid-Open No. 2004-304771, when signals from pixelsof three rows provided in the same column are to be added, the threesampling capacitors 1200 a to 1200 c are first made to hold signals ofrespective rows sequentially. Subsequently, the signals held in thesampling transistors 1630 a to 1630 c are added by turning on thesampling transistors 1630 a to 1630 c at the same time. Then, when thecolumn selection transistor 1650 is turned on, the signal correspondingto the sum of the three rows is read via a horizontal signal line 1570.At this time a gain G1 by which a signal read via the horizontal signalline 1570 is to be multiplied is given by the following, letting Csp/3be the capacitance value of the respective capacitors 1200 a to 1200 c,and Ccom be the capacitance value of the horizontal signal linecapacitor 1210.

G1=(Csp/3+Csp/3+Csp/3)/(Csp/3+Csp/3+Csp/3+Ccom)=Csp/(Csp+Ccom)  (1)

It is disclosed in Japanese Patent Laid-Open No. 2004-304771 thataddition of two rows is performed in the case where four samplingcapacitors are connected in parallel. As such a method, Japanese PatentLaid-Open No. 2004-304771 discloses a method in which a signal voltageof each row is stored in two sampling capacitors.

Thus, when two-row addition is to be performed, all the samplingcapacitors can be utilized in the case where the sampling capacitors areprovided in a number which is a multiple of two (four in the aboveexample). However, the following case is not considered: the case wherethe sampling capacitors are provided in a number which is not a multipleof the number of rows to be added, specifically, when addition of tworows is to be performed in the case where three sampling capacitors areprovided.

SUMMARY OF THE INVENTION

The present invention advantageously provides sampling capacitors in asolid-state image pickup apparatus having a plurality of addition modesfor adding signals of a different number of pixels.

A solid-state image pickup apparatus according to an aspect of thepresent embodiment includes a plurality of pixels, a reading unit towhich the plurality of the pixels are connected and which holds signalsfrom the pixels, and a control unit capable of controlling operations ofthe plurality of the pixels and the reading unit. The control unitcontrols the plurality of the pixels and the reading unit in a firstoperation mode in which addition is not performed, in a second operationmode in which signals from aa of the plurality of the pixels are added,aa being an integer greater than one, and in a third operation mode inwhich signals from bb of the plurality of the pixels are added, bb beingan integer greater than aa. The reading unit includes a holding unithaving a capacitance value of C. The holding unit includes a firstcapacitor having a capacitance value of C/bb and a second capacitorhaving a capacitance value of C/p, p being a common multiple of aa andbb. The control unit, in the second operation mode, controls the firstand second capacitors such that groups each constituted of one of thefirst capacitors and more than one of the second capacitors orconstituted of a plurality of the second capacitors each have acapacitance value of C/aa, and the control unit causes the groups toeach hold the respective signal from the pixel. The control unit, in thethird operation mode, causes groups each constituted of p/bb of thesecond capacitors to each hold the respective signal from the pixel andcauses the first capacitors to each hold the respective signal from thepixel.

According to the present invention, it is possible to advantageouslyprovide sampling capacitors in a solid-state image pickup apparatushaving a plurality of addition modes for adding signals of a differentnumber of pixels.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a schematic configuration of asolid-state image pickup apparatus according to an embodiment of thepresent invention.

FIG. 2 is an illustration showing an example configuration of a signalprocessing unit according to a first embodiment of the presentinvention.

FIG. 3 is a timing diagram showing an example of driving timingaccording to the first embodiment.

FIG. 4 is a timing diagram showing an example of driving timingaccording to the first embodiment.

FIG. 5 is a timing diagram showing an example of driving timingaccording to the first embodiment.

FIG. 6 is an illustration showing a schematic configuration of asolid-state image pickup apparatus according to a second embodiment ofthe present invention.

FIG. 7 is an illustration showing an example configuration of a spatialposition of a signal and a color filter array according to the secondembodiment.

FIG. 8 is an illustration showing an example configuration of a signalprocessing unit according to the second embodiment.

FIG. 9 is a timing diagram showing an example of driving timingaccording to the second embodiment.

FIG. 10 is a timing diagram showing an example of driving timingaccording to the second embodiment.

FIG. 11 is an illustration showing an example configuration of a signalprocessing unit according to a third embodiment of the presentinvention.

FIG. 12 is a timing diagram showing an example of driving timingaccording to the third embodiment.

FIG. 13 is a timing diagram showing an example of driving timingaccording to the third embodiment.

FIG. 14 is a timing diagram showing an example of driving timingaccording to the third embodiment.

FIG. 15 is an illustration showing an example configuration of an imagepickup system according to a fourth embodiment of the present invention.

FIG. 16 is the circuit configuration diagram of the signal processingunit according to Japanese Patent Laid-Open No. 2004-304771.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments according to the present inventionare described.

Generally, “averaging” and “addition” of signals are considered to havethe same meaning. In the description of the following embodiments,although “averaging” and “addition of charges” are distinguished, theseare generally referred to as pixel addition.

Before the embodiments according to the present invention are explained,issues that arise in the solid-state image pickup apparatus disclosed inJapanese Patent Laid-Open No. 2004-304771 will be explained. Forinstance, suppose that a solid-state image pickup apparatus having thesignal processing circuit shown in FIG. 16 has a two-row addition modein which signals from pixels of two rows are added and a three-rowaddition mode in which signals from pixels of three rows are added. Thethree-row addition mode can be realized by the operation explainedabove. The following method may be considered as a method to realize thetwo-row addition mode using this signal processing circuit. First, thesampling capacitors 1200 a are caused to hold signals from the pixels ofthe first row, and then the sampling capacitors 1200 b are caused tohold signals from the pixels of the second row. Subsequently, by turningon the sampling transistors 1630 a and 1630 b at the same time, thesignals held in the sampling capacitors 1200 a and 1200 b are added, andthe signals obtained by the addition are read by the horizontal signalline 1570 without connecting the sampling capacitors 1200 c which do nothold signals. In this case, a gain G2 by which the signals read by thehorizontal signal line 1570 is to be multiplied is given by thefollowing.

G2=(Csp/3+Csp/3)/(Csp/3+Csp/3+Ccom)  (2)

The ratio of G2 and G1 is given by the following, using Equations (1)and (2).

G2/G1={2(Csp+Ccom)}/{2Csp+3Ccom}<1  (3)

Thus, the gain for the two-row addition mode is lower than the gain forthe three-row addition mode. The decrease in gain means a decrease inthe amplitude of the signal obtained at an output OUT, causing adecrease in the ratio of signal and noise, namely, a S/N ratio. Thedecrease in the S/N ratio causes worsening of a low light intensitylimit, which indicates a lower limit of light intensity at whichphotographing of a low light intensity object is allowed while keeping apredetermined S/N ratio. As can be seen from the above equation, thecapacitance values of the sampling capacitors need to be increased inorder to suppress the decrease in gain in the pixel addition operation;however, this will cause the area of the chip of a solid-state imagepickup apparatus to increase. When the capacitance values of therespective sampling capacitors 1200 are increased, the decrease in gainin reading of signals from the sampling capacitors 1200 to be output tothe horizontal signal line 1570 can be suppressed; however, since thesampling capacitors 1200 c are not utilized in the two-row additionmode, the gain for the two-row addition mode becomes lower than for thethree-row addition mode.

A first embodiment according to the present embodiment will be explainedtaking as an example a solid-state image pickup apparatus which operatesin the following three operation modes: a first operation mode in whichall pixels are read, a second operation mode which is a two-row additionmode, and a third operation mode which is a three-row addition mode. Forease of understanding, a monochrome solid-state image pickup apparatusis assumed.

FIG. 1 shows a schematic configuration of a solid-state image pickupapparatus according to the first embodiment of the present invention.Pixels 500 are arranged in a matrix in a pixel area 510. Here a pixelarea having three rows×three columns is shown; however, this does notlimit the size of the pixel area. In the respective pixels 500, signalsfrom the pixels 500 are output to respective first vertical signal lines530 by being driven by, for example, signals READ, RESET, and LSETsupplied from a row selection circuit 520 which is a control unit. Thesignals output to the first vertical signal lines 530, after beingsubjected to processing by a signal processing unit 100 which is areading unit, are transmitted to an output amplifier 580 via ahorizontal signal line 570 and output from an output terminal OUT. Acolumn selection circuit 560 which is the control unit supplies a signalfor selecting columns included in the signal processing unit 100.

Configurations of the pixels 500 will now be explained. Each of thepixels 500 included in the pixel area 510 includes a photodiode 501, atransfer transistor 502, a reset transistor 504, a floating diffusion(FD) 506, an amplifier transistor 503, and a selection transistor 505.The photodiode 501, which is a photoelectric conversion unit, generatesand stores charge in accordance with an amount of incident light. Thetransfer transistor 502, which is a transfer unit, switches betweenconnecting and disconnecting the photodiode 501 and the FD 506. A gateelectrode of the amplifier transistor 503, which is a pixel output unit,is connected to the FD 506. The reset transistor 504, which is a resetunit, switches between connecting and disconnecting a power supply VDDand the gate terminal of the amplifier transistor 503. When both of thetransfer transistor 502 and the reset transistor 504 are turned on atthe same time, the photodiode 501 is reset by the power supply VDD. Theamplifier transistor 503 forms a source follower circuit together with aconstant current supply (not shown) included in a load transistor unit540 while the selection transistor 505, which is a pixel selection unit,is turned on. On the first vertical signal line 530, which is a firstsignal line, a potential level according to the gate potential of theamplifier transistor 503 at that time appears. The READ signal fordriving the transfer transistor 502, the RESET signal for driving thereset transistor 504, and the LSET signal for driving the selectiontransistor 505 are commonly supplied to a plurality of pixels includedin the same row from the row selection circuit 520.

FIG. 2 illustrates an exemplary configuration showing more specificallythe signal processing unit 100 which is the reading unit. Here, a numberof pixels corresponding to columns m and (m+1) have been extracted. Thesignal processing unit 100 includes, for each of the pixel columns, asample hold transistor 600, a sampling capacitor 200, a clamp capacitor610, a second vertical signal line 620, a capacitor selection switch630, a clamp switch 640, and a column selection switch 650. The firstvertical signal line 530 is connected to the second vertical signal line620 via the sample hold transistor 600 and the clamp capacitor 610.Connected to the second vertical signal line 620 are sampling capacitors200 a to 200 d via the capacitor selection switches 630 a to 630 d.Further the clamp switch 640 and the column selection switch 650 areconnected to the second vertical signal line 620, which is connected tothe horizontal signal line 570 via the column selection switch 650. Ahorizontal signal line capacitor 210 corresponds to the capacitance ofthe horizontal signal line 570.

When the capacitor selection switches 630 a to 630 d are all on, thesampling capacitors 200 a to 200 d are electrically connected inparallel. Supposing that the capacitance value at this time is a firstcapacitance value Csp (also denoted by C), each of the samplingcapacitors 200 a and 200 d has a capacitance value Csp/3, and each ofthe sampling capacitors 200 b and 200 c has a capacitance value of Cs/6.

Referring to FIG. 2, signals SP, SW1 to SW4, and CP are supplied, forexample, from a timing control circuit explained later.

The operation of reading the signals using a solid-state image pickupapparatus having the configuration shown in FIGS. 1 and 2 will now beexplained.

All-Pixel Reading Mode

First, an operation mode in which pixel addition is not performed willbe explained with reference to FIG. 3. Here, it is assumed that thesignals SW1 to SW4 supplied to the capacitor selection switches 630 a to630 d are always at a high level. In other words, this is a state wherea capacitor having the first capacitance value Csp is connected to thesecond vertical signal line 620. Note that for ease of understanding,the second vertical signal line 620 is assumed to have negligiblecapacitance.

A period “row n” in each column is considered. When a signal LSE/n)becomes a high level at time t1, the selection transistor 505 includedin the pixel of row n is turned on and performs a source followeroperation, and hence the potential of the gate of the amplifiertransistor 503, i.e., a potential level corresponding to the potentialof the FD 506 appears on the first vertical signal line 530. In otherwords, a state in which the pixel of row n is selected starts at timet1. At the same time, since the signals SP and CLP also become a highlevel at time t1, the clamp capacitor 610 enters a state in which apotential difference between the potential level appearing on the firstvertical signal line 530 and a voltage CPDC is given.

When the signal RESET (n) becomes a high level in the form of a pulsefrom time t2, the potential of the FD 506 is reset in accordance withthe voltage of the power supply VDD. Consequently, a level correspondingto the resetting of the FD 506 appears on the first vertical signal line530 and a terminal A of the clamp capacitor 610.

When the signal CLP becomes a low level at time t3, the clamp switch 640is turned off, and hence, the second vertical signal line 620 enters anelectrically floating state. Consequently, the clamp capacitor 610 holdsthe potential difference between the voltage level corresponding to theresetting of the FD 506 and the voltage CPDC.

When the signal READ(n) becomes a high level from time t4, the chargestored in the photodiode 501 is transferred to the FD 506. The potentialof the FD 506 changes in accordance with the transferred charge, a levelcorresponding to this appears on the first vertical signal line 530. Theclamp capacitor 610 continues to hold a potential difference between alevel corresponding to the resetting of the FD 506 and the voltage CPDC.Hence, the potential of the second vertical signal line 620 changes byan amount which is a difference ΔVin between a level corresponding tothe resetting of the FD 506 and a level corresponding to the transfer ofthe charge from the photodiode 501 to the FD 506, multiplied by a gaindetermined by the ratio of the capacitance values.

Supposing that the capacitance value of the clamp capacitor 610 is Ccp,the voltage change ΔV generated on the second vertical signal line 620is given by the following.

ΔV=(Ccp/Csp)××Vin  (4)

The level corresponding to the resetting of the FD 506 includes a noisecomponent due to the switching of the reset transistor 504 and a noisecomponent specific to the transistor that constitutes a pixel. The leveldue to the transfer of charge from the photodiode 501 to the FD 506 alsohas this noise component superimposed thereon. Hence, by performing theabove-described operation using the clamp capacitor 610, noisecomponents are decreased.

When the signal SP becomes a low level at time t5, the sample holdtransistor 600 is turned off, whereby the clamp capacitor 610 and thefirst vertical signal line 530 are electrically disconnected.Consequently, ΔV is held in the sampling capacitors 200 a to 200 d.

When the signal LSE/n) becomes a low level at time t6, the amplifiertransistor 503 included in the pixel 500 of row n and the first verticalsignal line 530 are electrically disconnected, whereby the sourcefollower operation ends. In other words, the state in which the pixel ofrow n is selected ends.

The same operations explained above are performed in parallel in each ofthe columns.

When signals CSEL (m) and CSEL (m+1) are supplied sequentially from timet7, the signals held in the sampling capacitors 200 a to 200 d ofrespective columns are output to the horizontal signal line 570. Whenthe signals held in the sampling capacitors 200 a to 200 d are output tothe horizontal signal line 570, the signals are multiplied by the gaindetermined by the capacitance ratio. Here, let the capacitance value ofthe horizontal capacitor 210 be Ccom and the gain of the outputamplifier 580 be Gamp. Then an output Vout at the output terminal OUT isgiven by the following.

Vout=ΔV×{Csp/(Csp+Ccom)}×Gamp  (5)

Similar operations are performed during periods “row n+1”, “row n+2”, .. . .

Two-Row Addition Mode

An operation mode for adding signals from two rows will now beexplained.

FIG. 4 shows an exemplary driving pattern in the two-row addition mode.Here, operations different from those in the all-pixel reading modeshown in FIG. 3 will be mainly explained. In the all-pixel reading mode,the sampling capacitors 200 a to 200 d are treated as one capacitor, inwhich the signal from one pixel is held. However, the two-row additionmode is different from the all-pixel reading mode in that the samplingcapacitors 200 a to 200 d are utilized by being divided into two groups.

In the period “row n”, in each column, shown in FIG. 4, the signals SW3and SW4 are kept at a low level, whereby the signal from the pixel ofrow n is held in the sampling capacitors 200 a and 200 b. In the period“row n+1”, the signals SW1 and SW2 are kept at a low level, whereby thesignal from the pixel of row n+1 is held in the sampling capacitors 200c and 200 d. In other words, the signal from the pixel of row n and thesignal from the pixel of row n+1 are respectively held in capacitorshaving the same capacitance value Csp/3+Csp/6=Csp/2.

Unlike the operation of the all-pixel reading mode shown in FIG. 3, theperiod “n+1” is followed by a period “addition”. In the two-row additionmode, the signals SW1 to SW4 become a high level at the same time duringthe period “addition”, whereby the sampling capacitors 200 a to 200 dare electrically connected. Consequently, the signal from the pixel ofrow n held in the sampling capacitors 200 a and 200 b and the signalfrom the pixel of row n+1 held in the sampling capacitors 200 c and 200d are added.

Then the signals CSEL are sequentially supplied while the signals SW1 toSW4 are at a high level, whereby the signals each corresponding to tworows are sequentially output to the horizontal signal line 570.

Next, a gain that the signal is multiplied by will be described. Let thesignal from the pixel of row n held in the sampling capacitors 200 a and200 b be ΔV_(n) and the signal from the pixel of row n+1 held in thesampling capacitors 200 c and 200 d be ΔV_(n+1). Then the voltage ΔV2 ofthe second vertical signal line 620 during the period “addition” isgiven by the following.

ΔV2={(Csp/3+Csp/6)×ΔV _(n)+(Csp/3+Csp/6)×ΔV_(n+1)}/{2×(Csp/3+Csp/6)}=(ΔV _(n) +ΔV _(n+1))/2  (6)

This corresponds to averaging of the signals from the pixelscorresponding to two rows. An output voltage ΔVout subsequently outputfrom the output terminal OUT via the horizontal signal line 570 and theoutput amplifier 580 is given by the following.

ΔVout=ΔV2×{Csp/(Csp+Ccom)}×Gamp  (7)

As can be seen from Equations (5) and (7), the gain for the voltage ΔV2appearing on the second vertical signal line 620 is{Csp/(Csp+Ccom)}×Gamp, which is the same as that in the all-pixelreading mode.

Three-Row Addition Mode

Next, an operation mode for adding signals from three rows will beexplained.

FIG. 5 shows an exemplary driving pattern in the three-row addition modein each column. Here, operations different from those in the all-pixelreading mode shown in FIG. 3 will be mainly explained. In the all-pixelreading mode, the sampling capacitors 200 a to 200 d are treated as onecapacitor, in which the signal from one pixel is held. However, thethree-row addition mode is different from the all-pixel reading mode inthat the sampling capacitors 200 a to 200 d are utilized by beingdivided into three groups.

In the period “row n” shown in FIG. 5, the signals SW2, SW3 and SW4 arekept at a low level, whereby the signal from the pixel of row n is heldonly in the sampling capacitor 200 a. In the period “row n+1”, thesignals SW1 and SW4 are kept at a low level, whereby the signal from thepixel of row n+1 is held in the sampling capacitors 200 b and 200 c.Further, during a period “row n+2”, since the signals SW1, SW2 and SW3are kept at a low level, the signal from the pixel of row n+2 is held inthe sampling capacitor 200 d. In other words, the signals from thepixels of rows n to n+2 are respectively held in capacitors having thesame capacitance value Csp/3.

During the period “addition” following the period “row n+2”, the signalsSW1 to SW4 become a high level at the same time, whereby the samplingcapacitors 200 a to 200 d are electrically connected. Consequently, thesignal from the pixel of row n held in the sampling capacitor 200 a, thesignal from the pixel of row n+1 held in the sampling capacitors 200 band 200 c, and the signal from the pixel of row n+2 held in the samplingcapacitor 200 d are added.

Then the signals CSEL are sequentially supplied while the signals SW1 toSW4 are at a high level, whereby the signals each corresponding to threerows are sequentially output to the horizontal signal line 570.

Next, a gain that the signal is multiplied by will be described. Let thesignal from the pixel of row n held in the sampling capacitors 200 a beΔV_(n) and the signal from the pixel of row n+1 held in the samplingcapacitors 200 b and 200 c be ΔV_(n+1). Similarly, let the signal fromthe pixel of row n+2 held in the sampling capacitor 200 d be ΔV_(n+2).Then the voltage ΔV3 of the second vertical signal line 620 during theperiod “addition” is given by the following.

ΔV3={(Csp/3)×ΔV _(n)+(Csp/6+Csp/6)×ΔV _(n+1)+(Csp/3)×ΔV_(n+2)}/{3×(Csp/3)}=(ΔV _(n) +ΔV _(n+1) +ΔV _(n+2))/3  (8)

This corresponds to averaging of the signals from the pixelscorresponding to three rows. An output voltage ΔVout subsequently outputfrom the output terminal OUT via the horizontal signal line 570 and theoutput amplifier 580 is given by the following.

ΔVout=ΔV3×{Csp/(Csp+Ccom)}×Gamp  (9)

As can be seen from Equations (5), (7), and (9), the gain for thevoltage ΔV3 appearing on the second vertical signal line 620 is{Csp/(Csp+Ccom)}×Gamp, which is the same as those in the all-pixelreading mode and two-row addition mode. In other words, in a solid-stateimage pickup apparatus having different addition modes, such as atwo-row addition mode and a three-row addition mode, the same gain isrealized for the different addition modes. Consequently, the issue inthe known technique is solved, and a decrease in the S/N ratio at thetime of addition is suppressed, while suppressing an increase in thechip size.

An example having a two-row addition mode and a three-row addition modehas been described above; however, the number of pixels to be added inaddition modes is not limited to this. To generalize the abovediscussion, a solid-state image pickup apparatus will be consideredwhich can operate in an aa-row addition mode for adding signals from aapixels and a bb-row addition mode for adding signals from bb pixels inaddition to an all-pixel reading mode for reading signals from pixelswithout performing addition. Here, it is assumed that aa is an integergreater than one, and bb is an integer satisfying a relation aa<bb.

Case 1) aa and bb are relatively prime, because they have no commonpositive factor other than 1 or their greatest common divisor is 1.

Let the capacitance value in the all-pixel reading mode be C. In thecase that aa and bb are relatively prime, the capacitors in the signalprocessing unit may be configured so as to include a first capacitorhaving a capacitance value of C/bb and a second capacitor having acapacitance value of C/p, where p is a common multiple of aa and bb. Thenumber of the first capacitors is equal to or more than one and equal toor less than (bb−1). The number of the second capacitors is equal to ormore than p/bb and equal to or less than {p−(p/bb)}. Further, a thirdcapacitor having a capacitance value of C/q may be included in additionto the first and second capacitors, where q is a common multiple of aaand bb, and is different from p. Specific numerical examples include acombination of: aa=2, bb=3, P=6, q=12. In this case, for example, one ofthe first capacitors having a capacitance value of C/3, two of thesecond capacitors having a capacitance value of C/6, and four of thethird capacitors having a capacitance value of C/12 may be provided as acombination.

The capacitors in the signal processing unit may be divided into firstcapacitors each having a capacitance value of C/bb and second capacitorseach having a capacitance value of C/(aa×bb). In this case, thecapacitance value of the second capacitor corresponds to the capacitancevalue described above in the case that p is the least common multiple ofaa and bb. When the first capacitors are provided in a number equal toaa and the second capacitors are provided in a number equal to(bb−aa)×aa, the total capacitance value is equal to C. The first andsecond capacitors thus configured allow the number of capacitors to besmaller than in the example described in the background.

In the aa-row addition mode, by combining one of the first capacitorsand (bb−aa) of the second capacitors as a group, groups each having acapacitance value of C/aa may be provided in a number equal to aa, andhence each group is made to hold a signal of one pixel.

When aa of the second capacitors are combined as a group, (bb−aa) ofthese groups may be provided. Since the capacitance value of the secondcapacitor is C/(aa×bb), this group has a capacitance value of C/bb. Inother words, this group can be treated as a capacitor same as the firstcapacitor. Hence, in the bb-row addition mode, by utilizing the firstcapacitor independently and the second capacitors as a group of aathereof, all capacitors can be handled as if capacitors having acapacitance value of C/b were provided in a number equal toaa+(bb−aa)=bb. Hence, a capacitor having a capacitance value C/bb or agroup of capacitors having a capacitance value C/bb is made to hold asignal of one pixel.

By configuring the first and second capacitors as described above, thefirst capacitance value C can be fully utilized in either of theaddition modes. Hence it is possible to suppress a decrease in the gainin a solid-state image pickup apparatus that can operate in twodifferent addition modes.

Case 2) aa and bb have a common divisor

Let the capacitance value in the all-pixel reading mode be C, as well.The case in which aa and bb have a common divisor can be restated as thecase in which relations n≠r×m, aa=m×c, and bb=n×c are satisfied, whereeach of c, m, n, and r is an integer.

When aa and bb have a common divisor, the capacitors in the signalprocessing unit may be configured so as to include a first capacitorhaving a capacitance value of C/bb and a second capacitor having acapacitance value of C/p, where p is a common multiple of aa and bb. Thenumber of the first capacitors is equal to or more than one and equal toor less than (bb−1). The number of the second capacitors is equal to ormore than m and equal to or less than (bb−1)×m. Further, a thirdcapacitor having a capacitance value of C/q may be included in additionto the first and second capacitors, where q is a common multiple of aaand bb, and is different from p. Specific numerical examples include acombination of: aa=6, bb=9, P=18, q=36. In this case two of the firstcapacitors having a capacitance value of C/9, six of the secondcapacitors having a capacitance value of C/18 and twelve of the thirdcapacitors having a capacitance value of C/36 may be provided as acombination.

The capacitors in the signal processing unit may be divided into firstcapacitors having a capacitance value of C/bb and second capacitorshaving a capacitance value of C/p, where p is the least common multipleof aa and bb. When the first capacitors are provided in a number equalto aa and the second capacitors are provided in a number equal to(bb−aa)×m, the total capacitance value is equal to the first capacitancevalue C. The first and second capacitors thus configured allow thenumber of capacitors to be smaller than in the example described in thebackground.

In the aa-row addition mode, by combining one of the first capacitorsand (n−m) of the second capacitors as a group, groups each having acapacitance value of C/aa may be provided in a number equal to aa, andhence each group is made to hold a signal of one pixel.

Considering that aa and bb satisfy the above relation, the capacitancevalue C/p of the second capacitors can be expressed by C/(m×n×c), sincep=m×n×c. When m of the second capacitors are combined as a group, thesegroups, having a capacitance value of C/(n×c)=C/bb, may be provided in anumber equal to (bb−aa). Hence, in the bb-row addition, each of the(bb−aa) groups including m of the second capacitors and each of the aafirst capacitors are made to hold a signal of one pixel.

By configuring the first and second capacitors as described above, thefirst capacitance value C can be fully utilized in either of theaddition modes. Hence it is possible to suppress a decrease in the gainin a solid-state image pickup apparatus that can operate in twodifferent addition modes.

The aa-addition mode and bb-addition mode have been described above, byclassifying various cases in accordance with the relation between aa andbb. In any of the cases described above, the common point is that theholding unit included in the signal processing unit includes the firstcapacitor having a capacitance value of C/bb and the second capacitorhaving a capacitance value of C/p (p is a common multiple of aa and bb).

According to the embodiment described above, it is possible to suppressa decrease in the S/N ratio during addition while suppressing anincrease in the chip area.

A second embodiment according to the present invention will be explainedusing an example of a solid-state image pickup apparatus which operatesin the following three operation modes: a first operation mode in whichall pixels are read, a second operation mode which is a two-row additionmode, and a third operation mode which is a three-row addition mode. Thesolid-state image pickup apparatus in the present embodiment is a colorsolid-state image pickup apparatus which is provided with a color filtercorresponding to each of the pixels and realizes weighted addition forpixels of the same row.

First, the advantage of performing weighted addition will be explained.FIG. 7C shows a generally used color filter array based on a Bayerarray. In FIG. 7C, R, G and B respectively represent filters that allowred, green, and blue light to pass therethrough. FIG. 7A shows row nextracted from the array. The suffixes indicate the positions of thepixels numbered starting from the left end. The figures in brackets showweights in addition. Generally, when addition of pixels is performed inthe direction along a row in a solid-state image pickup apparatusprovided with color filters, signals from neighboring pixels having thesame color are added. Hence, referring to FIG. 7A, addition is performedfor the combinations of G1 and G3, R2 and R4, G5 and G7, and R6 and R7,etc. Since the signals of the two pixels are added with a weightingratio of 1:1, the position of the spatial gravity center of the signalafter addition is a position shown by G2, which is between G1 and G3.Similarly, the spatial centers of gravity after addition appear at R3for R2 and R4, G6 for G5 and G 7, and R7 for R6 and R8. Hence, signalsof row n of an image created by an image signal processing circuit etc.,external to the solid-state image pickup apparatus have an arrangementshown on the right side of FIG. 7A. In other words, G and R afteraddition are not spatially positioned with equal spacing, and are in aneccentric arrangement in which G and R are close to each other. Sucheccentricity of the color center of gravity is not desired because itcauses aliasing when objects having high spatial frequencies arephotographed.

A method for lessening such eccentricity is weighted addition. Referringto FIG. 7B, the concept of weighted addition will be described. Inweighted addition, three neighboring R pixels are added with a weightingratio of 1:2:1. Consequently, the position of the spatial color centerof gravity of R after addition is a position shown by R4. Similarly,pixels at R6, R8, and R 10 are added with a weighting ratio of 1:2:1. Inother words, the pixels at R2, R4, and R 6 are added with a weightingratio of 1:2:1. On the other hand, G pixels are added with a weightingratio of 1:1. Hence, the signals of row n of an image created by animage signal processing circuit etc. external to the solid-state imagepickup apparatus have an arrangement shown on the right side of FIG. 7B,in which G pixels and R pixels are arranged with equal spacing. Thisadvantageously makes it unlikely that aliasing will occur.

FIG. 6 shows a schematic configuration of the solid-state image pickupapparatus according to the present embodiment. A notable difference fromthe solid-state image pickup apparatus shown in FIG. 1 is that variousunits such as load transistor units, signal processing units, columnselection circuits, and output amplifiers are provided so as to sandwicha pixel area 510. Signals from pixels of odd-numbered columns(hereinafter, called odd columns) are output from an output terminalOUT2 via a signal processing unit 110 b. Signals from pixels ofeven-numbered columns (hereinafter, called even columns) are output froman output terminal OUT1 via a signal processing unit 110 a. Filtershaving the same color are provided at every other pixel in a Bayerarray. Hence, when a certain row is considered, the output terminal OUT1only outputs signals from pixels having the same color, and similarly,the output terminal OUT2 only outputs signals from pixels having thesame color.

FIG. 8 shows an exemplary configuration of a signal processing unit 100functioning as a reading unit. Here, a portion corresponding tosame-color pixels of four columns have been extracted. Notabledifferences from the configuration shown in FIG. 2 are that inter-columnswitches 700, functioning as connection units, and column additionselection switches 710 have been added.

All-Pixel Reading Mode

Referring to FIG. 9, an operation mode in which pixel addition is notperformed will be explained. A signal HADD for driving the inter-columnswitches 700 and signals HSW1 and HSW2 for driving the column additionselection switches 710 have been added to the timing chart shown in FIG.3.

The operation performed in the all-pixel reading mode shown in FIG. 9 issimilar to that shown in FIG. 3 except that the signal HADD is kept at alow level and the signals HSW1 and HSW2 are kept at a high level. Ineach of the columns, signals from the respective pixels are held insampling capacitors 200 a to 200 d. Hence, similarly to the firstembodiment, a voltage change ΔV generated on a second vertical signalline 620 is output from the output terminal OUT after being multipliedby a gain of {Csp/(Csp+Ccom)}×Gamp. Here, Ccp is the capacitance valueof a clamp capacitor 610 and Ccom is the capacitance value of ahorizontal signal line 570.

Two-Row Addition Mode

The two-row addition mode will now be explained with reference to FIG.10. The difference from the timing diagram shown in FIG. 4 is thatalthough the period “n” is followed by the period “n+1” in FIG. 4, theperiod “n+1” is replaced with a period “n+2” in FIG. 9, and a signalfrom the pixel of row n+2 is read during the period “n+2”. This isbecause in the Bayer array, neighboring pixels on the same line havedifferent colors as shown in FIG. 7. In addition, the period “addition”in FIG. 4 has been replaced with a period “row addition”, a period“column addition”, and a period “output”.

The period “row n” in each column is considered. When a signal LSE/n)becomes a high level at time t1, a selection transistor 505 included inthe pixel of row n is turned on and performs a source followeroperation, and hence the potential of the gate of an amplifiertransistor 503, i.e., a potential level corresponding to the potentialof a FD 506 appears on a first vertical signal line 530. At the sametime, since signals SP and CLP become a high level at time t1, a clampcapacitor 610 enters a state in which it is given a potential differencebetween the potential level appearing on the first vertical signal line530 and a voltage CPDC. Since signals SW1 and SW2 also become a highlevel at time t1, sampling capacitors 200 a 1 and 200 b 1 enter a stateof being connected to a second vertical signal line 620. In other words,this is the same as the state in which a sampling capacitor having acapacitance value of Csp/3+Csp/6=Csp/2 is connected to the secondvertical signal line 620. Further, the signals HSW1 and HSW2 also becomea high level at time t1, whereby the column addition selection switch710 enters an on state.

When a signal RESET (n) becomes a high level in the form of a pulse fromtime t2, the potential of the FD 506 is reset in accordance with thevoltage of the power supply VDD. Consequently, a level corresponding tothe resetting of the FD 506 appears on the first vertical signal line530 and a terminal A of the clamp capacitor 610.

When the signal CLP becomes a low level at time t3, a clamp switch 640is turned off, and hence, the second vertical signal line 620 enters anelectrically floating state. Consequently, the clamp capacitor 610 holdsthe potential difference between the voltage level corresponding to theresetting of the FD 506 and the voltage CPDC.

When a signal READ(n) becomes a high level from time t4 in the form of apulse, the charge stored in a photodiode 501 is transferred to the FD506. The potential of the FD 506 changes in accordance with thetransferred charge, a level corresponding to this appears on the firstvertical signal lines 530. The clamp capacitor 610 continues to hold apotential difference between a level corresponding to the resetting ofthe FD 506 and the voltage CPDC. Hence, the potential of the secondvertical signal line 620 changes by an amount which is a difference ΔVinbetween a level corresponding to the resetting of the FD 506 and a levelcorresponding to the transfer of the charge from the photodiode 501 tothe FD 506 multiplied by a gain determined by the ratio of thecapacitance values. By letting the capacitance value of the clampcapacitor 610 be Ccp, the voltage change ΔV generated on the secondvertical signal line 620 is given by the following.

ΔV=(Ccp/Csp)×ΔVin  (10)

The level corresponding to the resetting of the FD 506 includes a noisecomponent due to the switching of a reset transistor 504 and a noisecomponent specific to the transistor that constitutes a pixel. The leveldue to the transfer of charge from the photodiode 501 to the FD 506 alsohas this noise component superimposed thereon. Hence, by performing theabove-described operation using the clamp capacitor 610, noisecomponents are decreased.

When the signal SP becomes a low level at time t5, the sample holdtransistor 600 is turned off, whereby a terminal A of the clampcapacitor 610 enters an electrically floating state.

When the signal LSE/n) becomes a low level at time t6, the amplifiertransistor 503 included in the pixel 500 of row n and the first verticalsignal line 530 are electrically disconnected, whereby the sourcefollower operation ends. The signals SW1 and SW2 also become a low levelat time t6, whereby ΔV is held in the sampling capacitors 200 a 1 and200 b 1. Although the signals LSE/n), SW1, and SW2 become a low level atthe same time in this diagram, the timing need not be the same.

The operation during the period “row n+2” is similar to the period “rown”. However, the signals LSET (n), RESET (n), and READ (n) are to berespectively replaced with signals LSET (n+2), RESET (n+2), and READ(n+2).

In the period “row addition” subsequent to the period “n+2”, the signalsSW1 to SW4 become a high level, whereby the sampling capacitors 200 a to200 d are electrically connected. Consequently, the signals held in thesampling capacitors 200 a and 200 b are added. In other words, theaverage level of the signals from the pixels of rows n and n+2 appearson the second vertical signal line 620.

During the period “column addition”, the signals SW1 to SW4 continue tobe kept at a high level as in the period “row addition”. When the signalHSW2 becomes a low level during the period “column addition”, the columnaddition selection switches 710 are turned off. Hence, a state isentered in which only the sampling capacitors 200 a and 200 b areelectrically connected to the second vertical signal lines 620 of theodd-numbered columns counted from the left of FIG. 8 (columns m, m+4,etc.). On the other hand, since the signal HSW1 is kept at a high level,the state is maintained in which the sampling capacitors 200 a to 200 dare electrically connected to the second vertical signal lines 620 ofthe even-numbered columns counted from the left of FIG. 8 (columns m+2,m+6, etc.). In other words, this state can be considered to be the statein which a capacitor having a capacitance value of Csp/2 is connected tothe second vertical signal lines 620 of the odd-numbered columns countedfrom the left, and a capacitor having a capacitance value of Csp isconnected to the second vertical signal lines 620 of the even-numberedcolumns counted from the left.

When the signal HADD becomes a high level during the period “columnaddition”, the inter-column switches 700 connect the second verticalsignal lines 620 of columns m to m+4. From the view point of samplingcapacitors, a state is entered in which the sampling capacitors 200 a 1and 200 b 1, 200 a 2 to 200 d 2, and 200 c 3 and 200 d 3 areelectrically connected. Hence, it becomes possible that the signals fromcolumns m, m+2, and m+4 are added with a ratio of 1:2:1 by making thesignal HADD at a high level. Further, similar structures are repeatedfor columns m+8 and more, although not shown. Hence, regarding columnsm+4, m+6, and m+8, for example, a state is entered in which the samplingcapacitors 200 a 3 and 200 b 3, 200 a 4 to 200 d 4, and 200 c 5 and 200d 5 are electrically connected.

During the period “output”, signals CSEL (m+2), CSEL (m+6) etc. aresupplied, whereby the signals from pixels of three neighboring columns,among two rows provided with the same color filters, are added with theweights of 1:2:1 in the direction of a row, and the resultant signal istransmitted to the horizontal signal line 570.

By letting the signal obtained by the 1:2:1 weighted addition be ΔV andthe gain of an output amplifier 580 be Gamp, a signal Vout output fromthe output amplifier 580 is given by the following.

Vout=ΔV>{Csp/(Csp+Ccom)}×Gamp  (11)

In other words, the gain is the same as that in the all-pixel readingmode.

In FIG. 10, the signal HADD becomes a low level in the period “output”;however the signal HADD may be set at a high level. When the signal HADDis set at a high level, the signal transferred to the horizontal signalline 570 corresponds to a signal held in a capacitor having acapacitance value of 2Csp. Hence, the gain for this signal is{2Csp/(Csp+Ccom)}×Gamp. In other words, a gain higher than that in theall-pixel reading mode can advantageously be applied. However, when thecapacitance value is increased, the time constant of a transientresponse which is determined by the stray resistance and capacitance ofthe circuit increases, thereby disadvantageously causing the operationspeed to be lowered. Hence, the signal HADD needs to be set inaccordance with this requirement.

The operation in the three-row addition mode is similar to the operationin the two-row addition mode except that the sampling capacitors 200 areutilized in such a manner as to be divided into three. Hence theexplanation thereof is omitted. Also in the three-row addition mode, thesignals held in the sampling capacitors are output after beingmultiplied by a gain of {Csp/(Csp+Ccom)}×Gamp.

Note that even when using the signal processing unit 100 having theconfiguration shown in FIG. 8, addition without weighting may beperformed. In this case, by always keeping the signals HSW1 and HSW2 ata high level and the signal HADD at a low level, this configuration canbe used as a configuration similar to that shown in FIG. 2.

An example having a two-row addition mode and a three-row addition modehas been described above; however, the number of pixels to be added inaddition modes is not limited to this. Generalization by using an aa-rowaddition mode and a bb-row addition mode is of course possible asdescribed in the first embodiment.

According to the embodiment described above, it is possible to suppressa decrease in the S/N ratio during addition while suppressing anincrease in the chip area.

In the first and second embodiments, examples were described in whichthe noise component due to pixels is decreased using a clamp capacity.In a third embodiment according to the present invention, aconfiguration will be described which can amplify a signal as well asdecrease the noise component due to pixels. The present embodiment willbe described also using an example of a solid-state image pickupapparatus which operates in the following three operation modes: a firstoperation mode in which all pixels are read, a second operation modewhich is a two-row addition mode, and a third operation mode which is athree-row addition mode.

FIG. 11 shows a configuration of one column extracted from a signalprocessing unit 100 functioning as a reading unit.

A column amplifier unit Amp includes a clamp capacitor 610, anoperational amplifier 660, a short circuit switch 670, and a feedbackcapacitor 680. One terminal A of the clamp capacitor 610 is connected toa first vertical signal line 530, and the other terminal B is connectedto the inverting terminal of the operational amplifier 660 and also toone terminal of the feedback capacitor 680 and one main electrode of theshort circuit switch 670. The other terminal of the feedback capacitor680 is connected to the other main electrode of the short circuit switch670 and the output terminal of the operational amplifier 660. Areference voltage VC0R is applied to the non-inverting terminal of theoperational amplifier 660. The short circuit switch 670 is controlled bya signal PC0R. The signal PC0R is supplied from, for example, a timingcontrol circuit described later.

The output of the operational amplifier 660, i.e., the output of thecolumn amplifier unit Amp is connected to a second vertical signal line620 s via a switch 700 and to a third vertical signal line 620 n via aswitch 701.

Sampling capacitors 200 as to 200 ds are connected to the secondvertical signal line 620 s via corresponding capacitor selectionswitches 630 as to 630 ds. Here, each of the sampling capacitors 200 asand 200 cs, and capacitors 200 an and 200 dn has a capacitance value ofCsp/3, and each of the sampling capacitors 200 bs and 200 cs, andsampling capacitors 200 bn and 200 cn has a capacitance value of Csp/6.The second vertical signal line 620 s is connected to a first horizontalsignal line 570 s via a column selection switch 650 s controlled by asignal CSEL(m). The first horizontal signal line 570 s has a capacitancevalue of Ccom. A horizontal capacitor 210 s schematically represents thecapacitance of the horizontal signal line 570 s. The first horizontalsignal line 570 s is connected to the non-inverting input terminal of adifferential amplifier 690, which is an output unit.

The sampling capacitors 200 an to 200 dn are connected to the thirdvertical signal line 620 n via corresponding capacitor selectionswitches 630 an to 630 dn. The third vertical signal line 620 n isconnected to a second horizontal signal line 570 n via a columnselection switch 650 n controlled by the signal CSEL(m). The secondhorizontal signal line 570 n has a capacitance value of Ccom. Ahorizontal capacitor 210 n schematically represents the capacitance ofthe second horizontal signal line 570 n. The second horizontal signalline 570 n is connected to the inverting input terminal of thedifferential amplifier 690, which is the output unit.

Referring to FIG. 12, an operation mode in which pixel addition is notperformed will now be described. Note that a pixel area connected to thefirst vertical signal lines 530 is the same as that shown in FIG. 1, anda monochrome solid-state image pickup apparatus is considered.

A period “row n” in each column is considered. When a signal LSE/n)becomes a high level at time t1, a selection transistor 505 included inthe pixel of row n is turned on and performs a source followeroperation, and hence the potential of the gate of an amplifiertransistor 503, i.e., a potential level corresponding to the potentialof a FD 506 appears on the first vertical signal line 530.

When a signal RESET (n) becomes a high level in the form of a pulse fromtime t2, the potential of an FD 506 is reset in accordance with thevoltage of the power supply VDD. Consequently, a level corresponding tothe resetting of the FD 506 appears on the first vertical signal line530 and a terminal A of the clamp capacitor 610.

When the signal PC0R becomes a high level in the form of a pulse at timet3, the short circuit switch 670 short-circuits the inverting inputterminal and output terminal of the operational amplifier 660. At thistime the voltage of the inverting terminal of the operational amplifier660 becomes a potential VC0R due to the virtual grounding of theoperational amplifier 660. In other words, both ends of the feedbackcapacitor 680 are reset by VC0R, and the voltage of the terminal B ofthe clamp capacitor 610 also becomes VC0R. Since signals SHS and SHN areat a low level at time3, the resetting is performed by the output of theoperational amplifier 660. When the signal PC0R becomes a low level, theterminal B of the clamp capacitor 610 enters an electrically floatingstate. Consequently, the clamp capacitor 610 holds the potentialdifference between the voltage level corresponding to the resetting ofthe FD 506 and the voltage VC0R.

When the signal SHN becomes a high level from time t4 in the form of apulse, the output of the column amplifier unit Amp is held in thesampling capacitors 200 an to 200 dn. The signals held in the samplingcapacitors 200 an to 200 dn include an output offset of the columnamplifier unit Amp.

When a signal READ(n) becomes a high level from time t5 in the form of apulse, the charge stored in a photodiode 501 is transferred to the FD506. The potential of the FD 506 changes in accordance with thetransferred charge, and a level corresponding to this appears on thefirst vertical signal line 530. The clamp capacitor 610 continues tohold a potential difference between a level corresponding to theresetting of the FD 506 and the voltage PC0R. Hence, the potential ofthe terminal B of the clamp capacitor 610 changes by an amount which isa difference ΔVin between a level corresponding to the resetting of theFD 506 and a level corresponding to the transfer of the charge from thephotodiode 501 to the FD 506. The difference from the first and secondembodiments is that ΔVin is multiplied by a gain determined by thecapacitance value ratio of the clamp capacitor 610 and the feedbackcapacitor 680. By letting the capacitance value of the clamp capacitor610 be C0 and the capacitance value of the feedback capacitor 680 be Cf,an output Ampout of the column amplifier unit Amp is given by thefollowing.

Ampout=(C0/Cf)×ΔVin  (12)

The level corresponding to the resetting of the FD 506 includes a noisecomponent due to the switching of a reset transistor 504 and a noisecomponent specific to the transistor that constitutes a pixel. Hence, byperforming the above-described operation using the clamp capacitor 610,noise components due to pixels are decreased. Further, the voltagechange ΔVin can be amplified with the ratio of C0/Cf, according to thepresent embodiment.

When the signal SHS becomes a high level at time t6 and subsequentlybecomes a low level, the voltage given by Equation (12) is held in thesampling capacitors 200 as to 200 ds. The signals held in the samplingcapacitors 200 as to 200 ds include an output offset of the columnamplifier unit Amp.

When the signal LSE/n) becomes a low level at time t7, the amplifiertransistor 503 included in the pixel 500 of row n and the first verticalsignal line 530 are electrically disconnected, whereby the sourcefollower operation ends. In other words, the state in which the pixel ofrow n is selected ends.

When signals CSEL (m) and CSEL (m+1) are supplied sequentially from timet8, the signals held in the sampling capacitors 200 as to 200 ds ofrespective columns are output to the horizontal signal line 570 s. Whenthe signals held in the sampling capacitors 200 as to 200 ds are outputto the horizontal signal line 570 s, the signals are multiplied by thegain determined by the capacitance ratio. By letting the capacitancevalue of the horizontal capacitor 210 s be Ccom, a voltage Vs1 appearingon the first horizontal signal line 570 s is given by the following.

Vs1={Csp/(Csp+Ccom)}×(C0/Cf)×ΔVin  (13)

Similarly, when the signals held in the sampling capacitors 200 an to200 dn are output to the horizontal signal line 570 n, the signals arealso multiplied by the gain determined by the capacitance ratio. Byletting the capacitance value of the horizontal capacitor 210 n be Ccom,a voltage Vn1 appearing on the first horizontal signal line 570 n isgiven by the following.

Vn1={Csp/(Csp+Ccom)}×(C0/Cf)×VC0R  (14)

The difference between Vs1 and Vs2 input to the differential amplifier690 is output from the output terminal OUT. Since Vs1 and Vs2 bothinclude the output offset of the column amplifier unit Amp, the outputterminal OUT outputs a signal in which the output offset of the columnamplifier unit Amp has been reduced. In other words, by letting Vout1 bea signal output from the output terminal OUT of the differentialamplifier 690, Vout1 is given by the following.

Vout1={Csp/(Csp+Ccom)}×(C0/Cf)×ΔVs3−{Csp/(Csp+Ccom)}×(C0/Cf)×ΔVn3={Csp/(Csp+Ccom)}×(C0/Cf)×(ΔVs3−ΔVn3)  (15)

Similar operations are performed during periods “row n+1” and “row n+2”.

Two-Row Addition Mode

An operation mode for adding signals from two rows will now bedescribed.

FIG. 13 shows an example driving pattern in the two-row addition mode.Here, operations different from those in the all-pixel reading modeshown in FIG. 12 will be mainly described. In the all-pixel readingmode, the sampling capacitors 200 an to 200 dn are treated as onecapacitor, and the sampling capacitors 200 as to 200 ds are treated asone capacitor. However, the two-row addition mode is different from theall-pixel reading mode in that the sampling capacitors 200 an to 200 dnare utilized by being divided into two groups and the samplingcapacitors 200 as to 200 ds are utilized by being divided into twogroups.

In the period “row n” in each column shown in FIG. 13, the signals SW3and SW4 are kept at a low level, whereby the signal from the pixel ofrow n is held in the sampling capacitors 200 as and 200 bs. In theperiod “row n+1”, the signals SW1 and SW2 are kept at a low level,whereby the signal from the pixel of row n+1 is held in the samplingcapacitors 200 cs and 200 ds. In other words, the signal from the pixelof row n and the signal from the pixel of row n+1 are respectively heldin capacitors having the same capacitance value Csp/3+Csp/6=Csp/2.

Different from the operation of the all-pixel reading mode shown in FIG.12, the period “n+1” is followed by a period “addition”. In the two-rowaddition mode, the signals SW1 to SW4 become a high level at the sametime during the period “addition”, whereby the sampling capacitors 200as to 200 ds are electrically connected. Consequently, the signal fromthe pixel of row n held in the sampling capacitors 200 as and 200 bs andthe signal from the pixel of row n+1 held in the sampling capacitors 200cs and 200 ds are added.

Then the signals CSEL are sequentially supplied while the signals SW1 toSW4 are at a high level, whereby the signals each corresponding to tworows are sequentially output to a horizontal signal line 570.

Next, a gain that the signal is multiplied by will be explained. Let thesignal from the pixel of row n held in the sampling capacitors 200 asand 200 bs be ΔV_(sn) and the signal from the pixel of row n+1 held inthe sampling capacitors 200 cs and 200 ds be ΔV_(sn+1). Then a voltageΔVs2 of the second vertical signal line 620 s during the period“addition” is given by the following.

ΔVs2={(Csp/3+Csp/6)×(C0/Cf)×ΔV _(sn)+(Csp/3+Csp/6)×(C0/Cf)×ΔV_(sn+1)}/{2×(Csp/3+Csp/6)}=(C0/Cf)×(ΔV _(sn) +ΔV _(sn+1))/2  (16)

A voltage ΔVn2 appearing on the third vertical signal line 620 n isgiven by a similar expression, and the output terminal OUT outputs asignal in which the output offset of the column amplifier unit Amp hasbeen reduced. In other words, a voltage Vout2 output from the outputterminal OUT of the differential amplifier 690 is given by thefollowing.

Vout2={Csp/(Csp+Ccom)}×(Co/Cf)×ΔVs2−{Csp/(Csp+Ccom)}×(Co/Cf)×ΔVn2={Csp/(Csp+Ccom)}×(Co/Cf)×(ΔVs2×ΔVn2)  (17)

Three-Row Addition Mode

Next, an operation mode for adding signals from three rows will bedescribed.

FIG. 14 shows an exemplary driving pattern in the three-row additionmode in each column. Here, operations different from those in theall-pixel reading mode shown in FIG. 12 will be described. In theall-pixel reading mode, the sampling capacitors 200 an to 200 dn aretreated as one capacitor, and the sampling capacitors 200 as to 200 dsare treated as one capacitor. However, the three-row addition mode isdifferent from the all-pixel reading mode in that the samplingcapacitors 200 an to 200 dn are utilized by being divided into threegroups and the sampling capacitors 200 as to 200 ds are utilized bybeing divided into three groups.

In the period “row n” shown in FIG. 14, the signals SW2, SW3 and SW4 arekept at a low level, whereby the signal from the pixel of row n is heldonly in the sampling capacitor 200 as. In the period “row n+1”, thesignals SW1 and SW4 are kept at a low level, whereby the signal from thepixel of row n+1 is held in the sampling capacitors 200 bs and 200 cs.Further, during a period “row n+2”, since the signals SW1, SW2 and SW3are kept at a low level, the signal from the pixel of row n+2 is held inthe sampling capacitors 200 ds. In other words, the signals from thepixels of rows n to n+2 are respectively held in capacitors having thesame capacitance value Csp/3.

During the period “addition” following the period “row n+2”, the signalsSW1 to SW4 become a high level at the same time, whereby the samplingcapacitors 200 as to 200 ds are electrically connected. Consequently,the signal from the pixel of row n held in the sampling capacitor 200 a,the signal from the pixel of row n+1 held in the sampling capacitors 200bs and 200 cs, and the signal from the pixel of row n+2 held in thesampling capacitor 200 ds are added.

Then the signals CSEL are sequentially supplied while the signals SW1 toSW4 are at a high level, whereby the signals each corresponding to threerows are sequentially output to the horizontal signal line 570 s.

Next, a gain that the signal is multiplied by will be explained. Let thesignal from the pixel of row n held in the sampling capacitors 200 as beΔV_(sn) and the signal from the pixel of row n+1 held in the samplingcapacitors 200 bs and 200 cs be ΔV_(sn+1). Similarly let the signal fromthe pixel of row n+2 held in the sampling capacitor 200 ds be ΔV_(sn+2).Then the voltage ΔVs3 of the second vertical signal line 620 s duringthe period “addition” is given by the following.

ΔVs3={(Csp/3)×(C0/Cf)×ΔV _(sn)+(Csp/6+Csp/6)×(C0/Cf)×ΔV_(sn+1)+(Csp/3)×(C0/Cf)×ΔV _(sn+2)}/{3×(Csp/3)}=(C0/Cf)×(ΔV _(sn) +ΔV_(sn+1) +ΔV _(sn+2))/3  (18)

This corresponds to averaging of the signals from the pixelscorresponding to three rows. The voltage ΔVn3 of the third verticalsignal line 620 n is given by a similar expression, and the outputterminal OUT of the differential amplifier 690 outputs a signal in whichthe output offset of the column amplifier unit Amp has been reduced. Inother words, a voltage Vout3 output from, the output terminal OUT of thedifferential amplifier 690 is given by the following.

Vout3={Csp/(Csp+Ccom)}×(Co/Cf)×ΔVs3−{Csp/(Csp+Ccom)}×(Co/Cf)×ΔVn3={Csp/(Csp+Ccom)}×(Co/Cf)×(ΔVs3−ΔVn3)  (19)

As can be seen from Equations (15), (17), and (19), the gain for thevoltage ΔV3 appearing on the second vertical signal line 620 is{Csp/(Csp+Ccom)}×(Co/Cf), which is the same as those in the all-pixelreading mode and two-row addition mode. In other words, in a solid-stateimage pickup apparatus having different addition modes, such as atwo-row addition mode and a three-row addition mode, the same gain isrealized for the different addition modes. Consequently, the issue inthe known technique is solved, and a decrease in the S/N ratio at thetime of addition is suppressed, while suppressing an increase in thechip size. Further, according to the present embodiment, since thecolumn amplifier unit Amp is provided, amplification can be performedwith a gain determined by the ratio of the clamp capacitor 610 and thefeedback capacitor 680. In the first and second embodiments, in which again of the ratio Ccp/Csp is applied between the clamp capacitor 610 andthe respective sampling capacitors 200, the signal becomes smaller,since the capacitance value of the sampling capacitor is generally setto a value larger than the capacitance value of the clamp capacitor. Onthe other hand, since the capacitance value of the feedback capacitor isset to a value smaller than the capacitance value of the clampcapacitor, the signal can be amplified with a gain of C0/Cf.

An example having a two-row addition mode and a three-row addition modehas been described above; however, the number of pixels to be added inaddition modes is not limited to this. Generalization by using an aa-rowaddition mode and a bb-row addition mode, as explained in the firstembodiment, may of course be employed.

As explained in the second embodiment, a configuration which allows forcolumn weighted addition may of course be employed.

According to the embodiment described above, it is possible to suppressa decrease in the S/N ratio during addition while suppressing anincrease in the chip area.

An overview of an image pickup system according to the presentembodiments will now be explained as a fourth embodiment with referenceto FIG. 15.

An image pickup system 800 includes, for example, an optical unit 810, asolid-state image pickup apparatus 1000, an image signal processingcircuit unit 830, a recording/communication unit 840, a timing controlcircuit unit 850, a system control circuit unit 860, and areproduction/display unit 870.

The optical unit 810, functioning as an optical system such as a lens,causes light from an object to form an image in a pixel portion, atwo-dimensional array of a plurality of pixels, of the solid-state imagepickup apparatus 1000. The pixel portion includes the valid pixel areadescribed above. The solid-state image pickup apparatus 1000 outputs asignal in accordance with the light of the image formed on the pixelportion, on the basis of the timing of a signal from the timing controlcircuit unit 850.

The signal output from the solid-state image pickup apparatus 1000 isinput to the image signal processing circuit unit 830 functioning as animage signal processing unit, and is subjected to processing such as ADconversion performed by the image signal processing circuit unit 830 inaccordance with a method defined by a program etc. The signal obtainedby the processing in the image signal processing circuit unit 830 issent to the recording/communication unit 840 as image data. Therecording/communication unit 840 sends a signal for forming an image tothe reproduction/display unit 870, to cause the reproduction/displayunit 870 to reproduce and display a movie or a still image. Therecording/communication unit 840 also communicates with the systemcontrol circuit unit 860 in response to the signal from the image signalprocessing circuit unit 830, and performs an operation of recording thesignal for forming an image on a recording medium (not shown).

The system control circuit unit 860 performs overall control of theimage pickup system 800, and controls the optical unit 810, the timingcontrol circuit unit 850, the recording/communication unit 840, anddriving of the reproduction/display unit 870. The system control circuitunit 860 is provided with a storage device (not shown) functioning as arecording medium, which records a program and the like necessary forcontrolling the operation of the image pickup system 800. The systemcontrol circuit unit 860 also supplies a signal for switching a drivingmode in the image pickup system 800 in accordance with an operation etc.of a user.

The timing control circuit unit 850 controls the driving timing of thesolid-state image pickup apparatus 1000 and the image signal processingcircuit unit 830 on the basis of the control performed by the systemcontrol circuit unit 860 functioning as a control unit.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications and equivalent structures and functions.

1. A solid-state image pickup apparatus, comprising: a plurality ofpixels; a reading unit to which the plurality of the pixels areconnected and which holds signals from the pixels; wherein the readingunit includes a first capacitor and a second capacitor, capacitancevalues of the first and the second capacitors being different from eachother.
 2. The solid-state image pickup apparatus according to claim 1,further comprising: a control unit configured to control operations ofthe plurality of the pixels and the reading unit, wherein the controlunit controls the plurality of the pixels and the reading unit in afirst adding operation mode in which signals from aa of the plurality ofthe pixels are added, aa being an integer greater than one, wherein thecontrol unit, in the first adding operation mode, controls the first andsecond capacitors such that the first capacitor and the second capacitorhold a signal derived from an identical one of the pixels, and whereinthe control unit, in the second operation mode, controls the first andsecond capacitors such that the first capacitor and the second capacitoreach holds a signal derived from different ones of the pixels.